1. Field of the Invention
This invention relates generally to an apparatus for an air conditioning system, and, in particular, to a thermostatic expansion valve.
2. Description of the Prior Art
A thermal, or thermostatic, expansion valve (TXV) is widely used in air conditioning systems to control the superheat at the evaporator outlet. A TXV throttles refrigerant and generates a hissing noise. This noise is especially prominent when the compressor is started, i.e., when the refrigerant flowing through the TXV has high vapor content or low sub-cooling and the flow rate has a transient peak because of the TXV dynamics and the resulting peak valve opening.
To resolve this problem, the size of the valve opening may be reduced by design, but this may unduly limit the cool-down performance of the system. Furthermore, it does not resolve the issue of rapid valve opening at the compressor startup and thus has limited effect according to experimental data.
Another solution to this problem is to add screens at the TXV inlets and outlets, but empirical evidence shows that such screens have only a limited effect in reducing the hiss noise.
Gradually opening the TXV allows more time for the high pressure side of the refrigerant loop to be pressurized, thereby reaching a more sub-cooled state, absorbing residual vapor, and reducing the initial refrigerant flow rate. As a result, the hissing noise through the thermostatic expansion valve shortly after compressor startup is minimized.
If the liquid inlet tube of the TXV has a substantial segment elevated relative to the TXV liquid inlet port, such that a substantial amount of refrigerant adjacent to the liquid inlet port remains liquid before the compressor startup or between the compressor startups, then the vapor stays at the peak portion of the inlet tube and the volume of the hissing noise may be reduced.
There is a need in the industry for a sequencing feature in a conventional TXV, which may be provided by a sequence valve that opens only after the system pressure differential reaches a predetermined value. In this case, substantial throttling of refrigerant starts only after the liquid line is substantially sub-cooled, and the effective throttling or metering opening either does not overshoot or remains to be small for a substantial period of time at compressor-on.